The present invention relates generally to heat exchangers for transferring heat from a gaseous heating fluid to a gaseous fluid to be heated and, more particularly, to such a gas to gas heat exchanger incorporating therein heat pipes, each heat pipe defining a working chamber having an evaporator section disposed in the flow path of the gaseous heating fluid and a condensor section disposed in the flow path of the gaseous fluid to be heated, and housing a working fluid which continuously undergoes a phase change as it circulates between the evaporator and condensor sections of the working chamber.
Heat exchangers incorporating numerous independently operating heat pipes to transfer heat from a hot gas stream to a cool gas stream have found wide application in industry. For example, in U.S. Pat. No. 2,813,698, heat pipes are used in a heat exchanger to preheat combustion air by transferring heat from the hot stack gas discharged from a furnace to combustion air being supplied to the furnace. Further, U.S. Pat. Nos. 4,616,697 and 4,687,649 disclose using heat pipe heat exchangers to reheat stack gas discharged from a wet scrubber by transferring heat from the hot stack gas upstream of the scrubber to the cool stack gas downstream of the scrubber.
In such heat exchangers, each heat pipe operates independently and consists of an elongated, closed tube which has been evacuated, filled with a heat transfer fluid, and hermetically sealed. Although many different heat transfer fluids have been successfully used as the working fluid in heat pipes, water is generally used as the working fluid in most industrial applications such as air preheating and flue gas reheating due to its low cost, ease of handling, safety and suitable heat transfer characteristics in the applicable range of operating temperatures for such application. In operation, the evaporator section of the heat pipe is disposed in the hot gas stream such that thermal energy is transferred from the hot gases to vaporize working fluid in the evaporator section. The vapor travels to the condensor section which is disposed in the gas stream be heated where the cool gas flowing over the heat pipe removes heat from the vapor causing the vapor to condense into liquid which flows back to the evaporator section of the heat pipe where it will be vaporized again by the hot flue gases. The closed loop evaporation-condensation cycle is continuous as long as there is a temperature difference between the combustion air or scrubber discharge gas to be heated and the flue gas serving as the heating fluid.
When used in heat exchangers for air preheating, flue gas reheating, and many other industrial gas to gas heat transfer applications, heat pipes are exposed to gas temperatures ranging from a low of about ambient temperature for the gas to be heated, to a typical high of about 150 C. to 200 C. for the heating gas, while the operating temperature of the heat pipes per se typically ranges from about 70 C. to about 130 C. depending from a heat pipes location within the heat exchanger with respect to the incoming flow of heating gas.
In any case, a problem generally experienced when using heat pipes to preheat combustion air or reheat flue gas via heat transfer from hot flue gas is that condensation of corrosive gases in the flue gas occurs when the flue gas temperature drops below the adiabatic saturation temperature of the flue gas. In combustion air preheat applications, condensation of corrosive gases usually occurs on the exterior surface of the evaporator portion of the heat pipes disposed in the flue gas flow at the cold end of the heat exchanger. However, as no corrosive gases are present in the combustion air being preheated, the condensor portions of these same heat pipes will not be exposed to corrosion. In flue gas reheat applications, the condensation of corrosive gases is usually experienced on the exterior surface of the condensor portion of the heat pipes disposed in the cold end of the heat exchanger in the flow of cool moisture-saturated flue gas discharged from the flue gas scrubber. However, as the hot flue gas serving as the heating gas in such flue gas reheat applications typically exits the heat exchanger at a temperature well above the saturation point, the evaporator portions of these same heat pipes will not be exposed to corrosion.
In conventional practice, it is customary to manufacture a heat pipe that is to be installed in the cold end of such a heat exchanger completely out of a highly corrosion resistant material having adequate strength properties, such as corrosion resistant steel alloys, or to coat the entire heat pipe with a layer or enamel of a corrosion resistant alloy or metal or plastic polymer. Although such a practice is indeed effective to protect the heat pipes installed in the cold end of such a heat exchanger against corrosion, it is an expensive practice as the entire heat pipe is customarily so treated even though only one portion of the heat pipe is actually ever exposed to corrosive elements.
Accordingly, in is an object of the present invention to provide a heat pipe which is manufactured from two separate tubular members, one of which may be produced from a material having relatively high corrosion resistance or from a material coated with a layer of corrosion resistant material, while the other portion may be produced from a less expensive material having relatively low corrosion resistance.